In North America, asphalt-based roofing materials are a popular medium for covering roofs on homes and other structures. Asphalt-based roofing materials come in shingle or roll form, the shingle being the more widely used material. A typical asphalt shingle or roll has an asphalt substrate and a multitude of roofing granules placed thereon. The roofing granules serve two purposes: (1) to protect the asphalt from exposure to ultraviolet rays; and (2) to provide color to the roofing material and ultimately to a roof.
Roofing granules come in a variety of colors ranging from white to black. Color has been provided to non-colored (raw) roofing granules in the following manner. First, raw granules of about 420-1680 micrometers (40 to 12 US mesh) are preheated to approximately 100.degree.-300.degree. F. (38.degree.-150.degree. C.). A paint slurry containing a pigment is then applied to the heated granules in a mixer. The color coated granules are then further heated in a kiln to about 350.degree. to 1200.degree. F. (175.degree.-650.degree. C.). This temperature may vary depending on the composition of the paint slurry. The granules are then cooled to approximately 210.degree.-240.degree. F. (100.degree.-115.degree. C.).
After being colored, the granules are passed to a post-treatment stage where the granules are treated with a solution of silicone and oil. The silicone/oil solution typically contains a petroleum oil and a mixture of various silicones. Typically, the silicones are derivatives of polydimethylsiloxane. The silicone/oil solution is applied to the colored roofing granules to reduce dust formation, to provide water repellency, and to promote adhesion of the granules to the asphalt-based substrate. After the silicone/oil treatment, the roofing granules are removed from the post-treatment stage, transported, and subsequently applied to an asphalt substrate.
When the roofing granules are secured to an asphalt-based substrate, there is a tendency for oils in the asphalt substrate to creep onto or be adsorbed on the granules' surfaces. This creeping or adsorption of the asphaltic oils on the roofing granules' surfaces causes a discoloration or staining of the roofing granules and hence the shingles. Although the discoloration is normally removed by weathering after the shingles are placed on a roof, there is nonetheless a disparity in color between what the consumer sees at the time of purchase and what the ultimate color of roof becomes after weathering. This color disparity is especially emphasized when the shingles have a lighter color, particularly white.
U.S. Pat. No. 2,927,045 discloses colored roofing granules that resist discoloration from oil. Raw granules are treated with a small amount of silicone during the colorization stage. The silicone is dispersed in a pigment composition, and the resulting composition is applied to the raw granules in a mixer. The coated granules are then transferred to a kiln where the granules are heated. Silicone compounds that contain long-chain alkyl or aryl groups are disclosed as being less desirable silicone additives. The long-chain organosilicones are less desirable because they tend to char when heated to elevated temperatures (column 3, lines 12--12).